Field of the Invention
The present invention relates to an image processing apparatus and an image processing method which convert image data to higher quality half-tone image data.
Description of the Related Art
Image forming apparatuses such as a printer have been widely used which print an image represented by digital image data input through a computer or a digital camera on a recording medium by using a recording material. In digital image data, pixel values of pixels included in image data are generally represented by multiple tones such as 8 bits or 16 bits for one color. On the other hand, in an image forming apparatus, an image is formed by ON and OFF states of dots with a recording material. Thus, the number of tones which are outputtable by the image forming apparatus is often lower than the number of tones of image data. Generally, half toning is performed on digital image data so that such an image forming apparatus may print an image. From the half-tone image data acquired by the half toning, the image forming apparatus represents a dot pattern to be output on a recording medium and thus represents pseudo-tones of the image.
A type of half toning is called a dither process using a threshold value matrix. The threshold value matrix refers to a matrix in which threshold values are arranged correspondingly to pixels in image data. The dither process is performed on each pixel included in image data by comparing the pixel value and a threshold value corresponding to the pixel to quantize the pixel value of each pixel and thus reduce the number of tones of image data. The threshold value matrix is arranged repetitively in a tile pattern on image data such that threshold values may correspond to all pixels.
Threshold value matrices usable for the dither process may be classified into two types. One type is an FM (frequency modulation) based threshold value matrix in which high and low threshold values are dispersed. The other type is an AM (Amplitude modulation) based threshold value matrix in which the magnitudes of threshold values increase sequentially from one position. In a case where the dither process is performed on image data by using an FM-based threshold value matrix, the resulting half-tone image data represent a dot pattern with dispersed dots and represent pseudo-tones with the number of dots per unit area. In a case where the dither process is performed on image data by using an AM-based threshold value matrix, the resulting half-tone image data represent a dot pattern in which a concentrated plurality of dots are periodically disposed and represent pseudo-tones in accordance with the areas of dots. Even with both of the threshold value matrices, when flat image data are input, the arrangement of threshold values are generally determined in consideration of output characteristics of the image forming apparatus.
However, comparing pixel values of pixels of input image data and corresponding threshold values, the resulting half-tone image data may not represent a desired dot pattern. This may be caused by interference between a characteristic exhibited by input image data and a characteristic of the threshold value matrix. For example, a tone represented by input image data of a partial region may not be reproduced, or a fine line to be reproduced with half-tone image data may have a break in it.
Accordingly, in Japanese Patent Laid-Open No. 2007-194904, an ideal output total value is generated for each region in accordance with an average value of pixel values of a plurality of pixels included in a region corresponding to a threshold value matrix. It is further disclosed that output values are allocated in order from a pixel at a centroid position within a region defined by a pixel value until an ideal output total value is acquired.
According to the disclosure of Japanese Patent Laid-Open No. 2007-194904, the arrangement of dots depends on the centroid position based on pixel values of pixels for each region. Therefore, because the centroid position within a region varies in accordance with a characteristic of input image data, a dot pattern suitable for output from an image forming apparatus may not be set. Furthermore, because the positions of ON dots are determined in order from the centroid position within a region, a characteristic in input image data may not be maintained, resulting in blurred half-tone image data.